Selecting optimal transmission in a centralized network

ABSTRACT

A method of determining a transmission process is disclosed. One embodiment comprises a method of notifying a central coordinator of a group to receive a transmission and obtaining system parameters from the central coordinator. The method then determine at least one merit parameter using at least one of the system parameters and at least one local parameter and selects a method of transmission based upon the merit parameter.

RELATED APPLICATION

[0001] This application is a continuation of U.S. ProvisionalApplication No. 60/414,149 and claims priority thereto.

FIELD

[0002] This disclosure relates to a point-to-point and/orpoint-to-multipoint networks with a centralized controller, moreparticularly to methods to allow broadcast and multicast in suchnetworks.

BACKGROUND

[0003] Some networks have a centralized controller that providesscheduled point-to-point communications, and may also coordinate acontention channel. These networks may also allow point-to-multipointcommunication links between devices, but only of limited capacity. Thesetypes of networks will be referred to as centralized networks. Incontrast, networks that have a shared medium, such as those having abroadcast channel, or those that have no centralized controller, such asInternet Protocol (IP) networks or other distributed networks operatingby an agreed-upon standard.

[0004] In centralized networks, communications between devices aregenerally scheduled and controlled by the central controller. If deviceA needs to send something to device B, device A must either send thecommunication to the central controller, which then sends it to deviceB, or device A must send a message to the central controller indicatingthe desire to send something to device B. In the latter example, thecentral controller then notifies all of the devices on the network tostay off the network at a certain point in time, as device A will beallowed to send the communication to device B at that time. Some systemsmay support a contention access channel, a first-come-first-servedchannel access for which the devices contend.

[0005] Because of the point-to-point nature of these systems, as well asthe need for a centralized controller, there are no current methods toallow for broadcast or multicast messages in these systems. A broadcastmessage is one sent to all devices on the network, while a multicastsystem is one sent to a specified subset of the network. Centralizednetworks may have the ability to send point-to-multipoint messages,which are in essence a multicast message, but the capabilities arelimited.

BRIEF DESCRIPTION OF THE DRAWINGS

[0006] The invention may be best understood by reading the disclosurewith reference to the drawings, wherein:

[0007]FIG. 1 shows an example of a centralized network.

[0008]FIG. 2 shows an example of channels in a centralized network.

[0009]FIG. 3 shows a flowchart of a method of transmitting data in acentralized network.

[0010]FIGS. 4a-4 c show flowcharts of embodiments of methods toestablish a channel for transmitting data, to transmit data in acentralized network and to release the channel.

[0011]FIG. 5 shows a flowchart of an alternative method of transmittingdata in a centralized network.

[0012]FIG. 6 shows a flowchart of an alternative method of transmittingdata in a centralized network.

[0013]FIG. 7 shows a flowchart of a method to determine a transmissionprocess.

[0014]FIG. 8 shows a block diagram of one embodiment of a centralcoordinator for a centralized network.

DETAILED DESCRIPTION OF THE EMBODIMENTS

[0015] As used here, the term ‘centralized network’ will be used torefer to networks having a central device called the Central Coordinatorto control bandwidth allocation to all devices within the network. Adata communication network using power line networks that exist in homesand buildings would be one example of such a network. However, themethods and apparatus disclosed here are relevant to any network thathave a centralized architecture with a central coordinator controlledthe activity of devices in the network.

[0016] A centralized network has two different types of entities,devices and a central coordinator (CC). Any device can function as thecentral coordinator provided it has the required capabilities. Theprocess of determining which device in the network functions as acentral coordinator is beyond the scope of this discussion. In thespecific example of a power line network (PLC), the devices, such asTVs, VCR, Computers, set-top boxes, home-audio equipment, etc.,communicate with each other via the network of power lines in thebuilding or home. However, other types of centralized networks exist andmay utilize embodiments of this invention. The use of a power linenetwork is only intended to aid in understanding of the invention.

[0017] An example of such a power line network is shown in FIG. 1. Allof the devices on the network are connected through the power lines inthe home, which form the power line network 10. A central coordinatorcontrols the access to, and use of, the network for communications. Inthis particular example, a security camera 14 at the front doortransmits video data across the PLC 10 to the destination devices 16 a,a first television, 16 b, a second television, and 16 c, a personalcomputer. In this particular example, the security camera is the sourcedevice and the TVs and the PC are the destination devices. Currently,the source device transmits the data by specifically identifying eachdestination device to the central coordinator 12 and setting up threeindividual channels to transmit those images. This is not a veryefficient design for a broadcast/multicast telecommunications system.

[0018] The design of telecommunications systems has traditionally beenbased on the Open Systems Interface (OSI) specification prescribed bythe International Standards Organization (ISO). The OSI model proposes apartitioning of functions of devices into 7 distinct protocol layers.Though the exact definition of these layers is open to interpretationthey represent a useful framework in which to discuss systemfunctionality. The 4 main layers of interest in a PLC system are theApplication Layer, Transport Layer, Medium Access layer and the PhysicalLayer.

[0019] The Application Layer includes both IP (Internet Protocol) andnon-IP based applications. Applications examples include Video such asHigh Definition Television (HDTV) and Standard Definition Television(SDTV), High quality audio, IP applications with Quality of Service(QoS) and other applications. Many of these applications require thatthe network posses some broadcast/multicast capabilities. An examplewould be Address Resolution Protocol (ARP) broadcasts in IP.

[0020] In an ARP broadcast, a first host is attempting to discover thephysical address of a second host, for which the first host has an IPaddress. The first host sends out a broadcast, which all devices on thenetwork receive. The second host with that IP address then replies withits physical address. In order for that type of discovery to work, thenetwork must support broadcast capabilities.

[0021] The Transport Layer consists of the protocols and methods thatare responsible for peer-peer transport of application data betweendevices. The chief function of the Transport Layer is the definition oflogical communication links or connections between peer entities andmanagement of the connection, including defining quality of service(QoS) parameters for application data and monitoring/enforcing the QoSparameters such as the average and maximum delay tolerable for eachpacket transmitted on the connection, bandwidth (BW) required, etc. Thetransport protocol in a PLC is essentially a connection-orientedprotocol as opposed to a packet oriented or connection-less protocols.Connections are defined to carry application data or control databetween peer-peer entities in the networks.

[0022] The Media Access Control (MAC) Layer provides functions requiredby the Transport System such as acknowledgements for reliable packetdelivery, in-sequence packet delivery, multiplexing of connections,concatenation and fragmentation of packets, etc. These functions will beused at the discretion of the Transport Layer manager. The physical(PHY) layer involves the digital signal processing systems for digitaltransfer of packets between devices. For the purpose of understandingembodiments of this invention, the discussion here is concerned onlywith a dual frequency-time division multiple access MAC/PHY protocolthat organizes time into units called Frames and sub-units called slotsand has a set of frequencies or “carriers” to assign to devices in aparticular Frame/slot. One such MAC/PHY protocol is OFDM or OrthogonalFrequency Division Multiplexing.

[0023] In an OFDM system, the central coordinator assigns bandwidth tothe devices by determining the Frames/slots in which each device ispermitted to transmit. The central coordinator also determines whatfrequencies/tones the device uses during the assigned slots as well asdigital communication parameters such as modulation density or number ofbits/symbol. This information is called an Allocation.

[0024] Several definitions for terms as used here will be helpful. Asused here, a device is a complete entity, having a full protocol stackas set out in the OSI model. A central coordinator is a specializeddevice within the network that maintains network timing, framestructure, network identity and allocation of bandwidth to connections.In addition, the central coordinator has a central bandwidth manager(CBWM) as well as other typical device functions. The centralcoordinator is a central repository of information and has globalknowledge of all connections and bandwidth allocated to each of theseconnections.

[0025] The CBWM is a function in the central coordinator that isresponsible for the allocation and management of Frequency and Timeassignments to channels or connections. A connection is a bi-directionalchannel between two devices, which is uniquely identified by itsConnection ID. A broadcast application is an application whose data istransmitted from a single source device to all devices on the network.Similarly, a multicast application is an application whose data istransmitted from a single source to a select subset of destinationdevices on the network.

[0026] The communication links between the different devices in thenetwork are called “channels”. Connections or logical traffic channelsbetween peer-peer Transport Layers may use one or more channels. It isassumed that the network provides for such channels between devices inthe network and between devices and the Central Coordinator (CC). Forpurposes of this discussion, four different channel types will be used:dedicated channel (D-CH), traffic channel (T-CH), beacon channel (B-CH)and the contention channel (C-CH).

[0027] The dedicated channel is used for communication between a deviceand the central coordinator. This channel is established when a devicefirst joins the network and stays alive for as long as the devices staysactive in the network. Within each frame, time slots and frequency tonesare reserved for a D-CH from every device to the central coordinator andfrom the central coordinator to every device.

[0028] Typically, the D-CH is a low bandwidth point-to-pointbi-directional link. The devices know the bandwidth allocated to theDedicated Channel before the broadcast/multicast actually takes place.This means that the bandwidth allocated to the D-CH is known at the timethe D-CH is originally established which is usually right after thedevice has registered with the central coordinator and has beenauthenticated and admitted to the network. The bandwidth of the D-CH isusually fixed right from the start, though this bandwidth can be changedby the CC. The D-CH is assumed to be a non-blocking channel wherepackets are queued if the D-CH is busy. The D-CH is also assumed toallow the data packets access to the D-CH when needed.

[0029] The contention channel (C-CH) uses all frequency tones in orderthat all devices can listen to it. Devices contend for the right totransmit on this channel. An example of a contention access protocol isSlotted ALOHA random access. Devices can use this channel on afirst-come-first-served basis. The central coordinator may advertise thebandwidth allocation (frequencies and times frames/slots) for acontention channel. Optionally, the central coordinator might alsoinform devices of the load on the Contention Channel. The C-CH is apoint-multipoint channel. In a multi-tone system where the channelcharacteristics of the communication link between any two devices canvary considerably, the modulation used on the C-CH has the leastspectral efficiency in order to allow all devices to receive thetransmissions successfully. This adversely affects the overallthroughput if this channel is used for data traffic.

[0030] The central coordinator uses the beacon channel (B-CH) toperiodically transmit network information and data. This channel usesall frequencies/tones in order that all devices can listen to it.Therefore, the spectral efficiency of the transmissions on this channelhas to be of the lowest order as explained in the case of C-CH. This isa very low bandwidth unidirectional, point to multi-point channel usedonly by the CC. The bandwidth of the B-CH is fixed, as are most of themessages that use this channel. The use of B-CH therefore is primarilyreserved for acknowledgements and not for application data transport.

[0031] Traffic channels (T-CH) are scheduled by the central coordinatorfor peer-peer communications in which a device can transmit data toanother device. The central coordinator might also schedule a trafficchannel with the central coordinator as the source. Traffic channels areestablished through an exchange of a request and response messages onthe Dedicated Channel between the central coordinator and the device.Each traffic channel is bi-directional and has a bi-directional“allocation” in terms of frequencies and time frames/slots. Only thedevice assigned the channel can transmit using this channel i.e. theallocation is dedicated to use by a single pair of devices.

[0032] Traffic channels can be point-point or point-multipoint channels.Since the channel characteristics between the source and destinationdevices on a traffic channel are estimated through the process ofchannel sounding, the spectral efficiency of the digital modulation onT-CH can be considerably higher than C-CH or B-CH. This enhances overallthroughput. T-CHs can be very high bandwidth channels as the centralcoordinator has the flexibility to allocate any of the availablebandwidth to the channel.

[0033] In determining how to do broadcast/multicast in such networks onemust consider all the channel structures available within the network.Each of the communication channels described earlier has its limitationsand positive attributes. Dedicated channels are fixed low bandwidthlinks but they are always present and hence eliminate the delayassociated with a request-grant channel set-up procedure. ContentionChannels do not need a request-grant procedure for channel set-up andhave low delay characteristics, provided there are few users contendingfor access to the channel. However, Contention Channels in systems suchas OFDM have to operate at the lowest modulation (bits/symbol) possiblein order for successful transmission of messages to all devices. Thisreduces efficiency of utilization of the available spectrum orthroughput. Further, if the probability of collisions increases,bandwidth is wasted and delay increases considerably.

[0034] Traffic channels are efficient for point-point communications.For point-multipoint operation traffic channels require a request-grantbandwidth allocation procedure that must also determines the rightmodulation and other physical channel parameters to enablecommunications from one source to many destinations. Whenever a channelsuch as C-CH or B-CH or a T-CH with all frequency tones is used forbroadcast, the modulation density that is used is typically the lowestpossible, so as to allow all devices to receive and decode the packetscorrectly. This reduces throughput, increases delay and decreases theefficiency of spectrum utilization.

[0035] A diagram of channels used in a power line communication systemsuch as that of FIG. 1 is shown in FIG. 2. The central coordinator has adedicated channel between it and all three devices, the source device Aand the destination devices B and C. A traffic channel is set up betweeneach device and the other devices. The designation of device A is onlytransitory and lasts only as long as A is transmitting data.

[0036] A first embodiment of a method for transmitting data is shown inFIG. 3. This embodiment will be referred to as the contention accessmethod. In this method, the source device uses the Contention Channel totransmit to all devices in the network. The use of C-CH allows thedevice to eliminate delay associated with a request-grant mechanismrequired for scheduled channels like the Traffic Channel. The C-CH alsoallows the source device to reach all devices in the network.

[0037] In FIG. 3, the flowchart is arranged so processes done by eachentity in the data transaction are identified as the source device, thecentral coordinator or the destination. For this method, the focus ismainly on the source device. An application, typically running on thesource device, generates the data to be broadcast to all devices in thenetwork at 20. Alternatively, the source device may identify a multicastset of destination nodes.

[0038] The application passes the data to be broadcast/multicast to theTransport/MAC Layer at 22. The MAC Layer achieves frequency and timesynchronization of the Frame and determines the position of thecontention channel within the Frame at 24, typically by listening to theFrame format broadcast by the central coordinator on the B-CH. Thesource device transmits the data over C-CH to the designated devices onPLC network at 26, and waits for an acknowledgment at 28.

[0039] When the central coordinator receives the data over thecontention channel, it acknowledges the successful reception of data 44through an ACK message generated at 46 that includes the equipmentidentifier of the source device. The ACK is transmitted at 48 over theB-CH, or the D-CH if it exists between the source device and the CC.This provides a degree of robustness to the C-CH transmissions sincesource device knows that at least one of its transmissions wassuccessful at 36 and it is likely the others were too, such as thatshown at 40. If the data were successful received at the destinationdevice at 40, the data would then be passed to the appropriateapplication running on the destination device at 42. This scheme doesnot guarantee delivery to all devices on the network or in the multicastgroup. Alternate acknowledgement schemes may possibly be used.

[0040] If the source device sends the data over the C-CH but it is notreceived successfully by the CC, no acknowledgment will be received at30. The source device may use a ‘back off’ algorithm to determine thenext time it will attempt the re-transmission and try to transmit thesame data again at 32. The source device can choose to re-broadcast thepackets multiple times up to a maximum retransmit count. The device canalso use timers to stop the re-transmissions once the timer expires. Thetimer may use Time to Live/Die values attached to the data packets ifthe system requires that this information be specified for all data.

[0041] In an alternative embodiment, the destination devices all respondwith an acknowledgement to the central coordinator at 41. The centralcoordinator then collects all of the acknowledgements at 43. The centralcoordinator is aware of all of the devices that were in the groupdesignated at 26. The central coordinator will generate a universalacknowledgement at 45, based upon the group. This may occur if all ofthe designated devices respond with acknowledgements, or if no responseis received from all devices within a predetermined period of time. Theuniversal acknowledgement is a list of all acknowledgements and negativeacknowledgements (NACKs). The source device then uses this informationin the decision at 32.

[0042] If the source device does not re-transmit the data at 32, theprocess records an error in transmission at 34. If the source devicedoes decide to re-transmit the data at 32, the process returns to thesynchronization at 24. At 38, the MAC layer informs the application ofeither success or failure.

[0043] Portions of an alternative method for transmitting data are shownin FIGS. 4a-4 c. In this method, the device explicitly requests thecentral coordinator for the establishment of a point-multipoint T-CH andwill be referred to here as a direct broadcast method. The focus isagain mainly on the source device. The request-grant exchange betweenthe central coordinator and the source device occurs on the D-CH. Thesource device may use C-CH also for sending requests upstream to the CC.This method had three phases: establishing the T-CH, transmitting dataon the T-CH, and releasing the T-CH.

[0044] An embodiment of establishing the traffic channel is shown inFIG. 4a, for situations in which no traffic channel exists. The processstarts at 50. At 52, an application running on the source devicerequests broadcast or multicast transmission, causing the device torequest a traffic channel at 54. The process waits to determine if thecentral coordinator has sent an acknowledgement at 56. Meanwhile, at541, the central coordinator receives the request. The centralcoordinator returns and acknowledgement to the source device at 542,received at the source device at 56. This is an acknowledgement of therequest, not that the channel has been established, which may bereferred to here as a channel acknowledgement. The traffic channel maybe unidirectional or bi-directional. A unidirectional channel has atraffic flow only from the source device to the destination devices, anda bi-directional channel has a traffic flow in both directions.

[0045] If the time for awaiting the acknowledgement of the request at 56is too long, the source device may retransmit the request. Thisdetermination is made at 561. If the decision to retransmit is made, theprocess returns to 52. If the decision is to not retransmit, the processmoves to 602 and the request fails. If the acknowledgement of therequest is received, the process then moves to 58 to await reply fromthe central coordinator as to whether the channel has been granted. Thedecision to re-transmit the request may be based upon expiration of atimer, or a predetermined request count being reached. The request maybe re-transmitted until an event occurs, where the event may be arequest acknowledgement being received, the timer expiring or therequest count being reached.

[0046] During the time that the source device is waiting betweenreceiving the request acknowledgement and the notification of channelgrant or not, the central coordinator performs admission control andbandwidth allocation processes at 544. If the system can grant thechannel at 544, the central coordinator informs the destination devicesof the channel at 545 and replies with an affirmative channel grant at546. At the source device, when the reply is received at 58, the devicethen determines that the channel has been granted at 60 and indicatesthe channel establishment status to the other layers in the sourcedevice.

[0047] If the channel is not granted at 544, the central coordinatorsends a negative request response to the source device at 58. Thedetermination that the channel was not granted is then made at 60. Thedevice may decide to re-try the channel request at 601. If the devicedoes not attempt to re-try the request, the request is indicated asfailed at 602. Again, the decision to re-try may be based uponexpiration of a timer or a predetermined re-try count being reached. There-try process may be set to repeat until an event occurs, where theevent is either a channel grant, expiration of the timer or a reaching are-try count.

[0048]FIG. 4b shows an embodiment of a method to transmit data on atraffic channel. The process starts at 66. Initially, the source devicedetermines whether a channel already exists at 68. If the channelexists, the source device then determines whether the channel is validat 70. If the answer to either of these two questions is ‘NO,’ theprocess goes to the establish channel procedure, such as the one shownin FIG. 4a. If the channel exists and is valid, the process moves to thesynchronization at 71, where the MAC layer achieves frequency and timesynchronization and determines the position of the traffic channelwithin the frame. Once the device is synchronized, it transmits data tothe destination devices and the central coordinator at 72.

[0049] In this embodiment, the central coordinator is used as a verifierto determine if the data has been successfully received. The destinationdevices receive the data at 721 and pass it to the relevant applicationon those devices at 722. In the case of a unidirectional channel, thedestination devices do not explicitly acknowledge reception. The centralcoordinator also receives the data at 723, but does generate anacknowledgement at 724 and transmits it back to the source device at725. As the traffic channel is unidirectional, this may be done on adedicated channel (D-CH) or the beacon channel (B-CH). Thisacknowledgement may be referred to here as a data acknowledgement. Thisallows the source device to determine that the data was successfullyreceived by at least one device in a unidirectional channel case, andtherefore uses that to conclude that the data transmission wassuccessful.

[0050] It is also possible that the channel established is abi-directional channel. In this case, the data acknowledgement processes724 and 725 may be performed by the central coordinator and anydestination device that successfully received the data.

[0051] As part of the process of waiting for the acknowledgement at 73,the source device may also have a timer that is set to expire within agiven time frame. If the timer expires, or an acknowledgement isreceived, the process moves to 74, where the success or failure of thetransmission is determined. If the acknowledgement was received, thetransmission was successful and that is sent back to the sendingapplication on the device at 75. If the transmission was not successfulat 74, the device either retransmits the data at 741, moving the processback to 68, or determines if there is more data to transmit at 76. Ifthere is more data to transmit at 76, the process returns to 68. Ifthere is no more data to transmit, the process moves to a releasechannel procedure, such as the one shown in FIG. 4c.

[0052] In FIG. 4c, the process to release the channel previouslyestablished starts at 78. The source device requests channel terminationat 79. The central coordinator receives the request at 791 and generatesand transmits the request acknowledgement at 792. The source deviceeither receives the acknowledgement at 80, or a predetermined timeperiod expires. If the acknowledgement is not received, the sourcedevice decides whether to retransmit the request at 801, which eithermoves the process back to 79 or fails the request at 822. If theacknowledgement is received at 80, the source device moves to a waitingstate at 81.

[0053] At the central coordinator, the channel is either terminated ornot at 793. If it is terminated, the destination devices are informed at794 and the affirmative reply to the request is sent at 795. Theaffirmative reply is received at 81, and the determination that thechannel has been released is made at 82. The release status is thenindicated back to the relevant applications on the source device at 83and the process ends at 84.

[0054] If the channel is not terminated at 793, a negative reply is sentto the source device at 796. The determination is then made that thechannel has not been released at 82. The device then determines whetherit should re-try at 821, sending the process back to 79, or fail therequest at 822. If the request fails at 822, the process then ends at84.

[0055] In another alternative method, referred to here as the dedicatedrelay method, the method assumes that every device has a bi-directionalD-CH channel between the central coordinator and the device. A flowchartof an embodiment of this method is shown in FIG. 5, with the focus beingmostly on the central coordinator. The D-CH channels and bandwidthallocations to these channels are established when the device is firstadmitted to the network and remain active as long as the device is apart of the network.

[0056] As discussed above with regard to FIGS. 4a-c, an application onthe device generates data at 90 and passes it to the MAC layer at 92.The MAC layer then synchronizes and transmits the data to the centralcoordinator at 96, waiting for the acknowledgment at 98. Similarprocesses for determining if the acknowledgment is received at 100, andwhether or not to re-transmit at 106 also occur. Once the acknowledgmentis received, the source device then waits for a status report from thecentral coordinator at 102. If the status is received at 102, the MAClayer then reports the status to the application at 104. If the statusis not received, the process fails at 108.

[0057] Different from the previous methods the source device transmitsthe data over the D-CH to be received by the central coordinator at 110,identifying the group of destinations for the data. When the centralcoordinator receives the data over the D-CH, it acknowledges thesuccessful receipt of the data over the D-CH at 112. At 114, the centralcoordinator transmits the data to the identified group of destinationsat 114. The central coordinator then waits for acknowledgments from eachdestination device at 116. If the acknowledgments are received at 118,the central coordinator generates a status report and sends it to thesource device. If no acknowledgment is received in the proper amount oftime or re-transmission, the central coordinator may decide tore-transmit at 122, returning the process to transmission at 114, ornote it as a failure at 124. If the transmission fails, this is thenreported as the status at 120. The status may take the form of theuniversal acknowledgement mentioned with reference to FIG. 3.

[0058] At the destination device, the data is received on each device'srespective dedicated channel between it and the central coordinator at126. Each destination then transmits an acknowledgment to the centralcoordinator at 128. While the central coordinator and source devicecontinue to communicate, the destination device forwards the data to theapplication for which it was intended at 130 and returns to normaloperations.

[0059] In an alternative relay method, shown in FIG. 6 and referred tohere as the dual channel relay method, the process is very similar tothat of the dedicated relay method, with the focus again being on thecentral coordinator. The source device transmits the data to the centralcoordinator at 96, in this case over either the dedicated channel or analready established traffic channel between the source device and theCC. When the central coordinator transmits its acknowledgment to thesource device it uses the same channel. For example, if the data isreceived on the dedicated channel, the acknowledgment is transmitted onthe dedicated channel.

[0060] The transmission and waiting processes on the source device areotherwise very similar to those of the dedicated relay method and willtherefore not be addressed again here. One of the main differencesbetween the dual channel relay method and the previously discusseddedicated relay method occurs at the central coordinator. Once theacknowledgment of the data is generated, the central coordinator thenestablishes a point to multi-point traffic channel at 132 and informsall of the destinations of the channel at 134. In an alternativeembodiment the central coordinator may establish individual,point-to-point traffic channels between it and each device. The datatransmission that occurs at 114, as well as the data reception at 126,may happen over the traffic channel, rather than the dedicated channel.The acknowledgment may occur over the dedicated channel between thedestination device and the central coordinator. The process continues onall three entities in a manner similar to the operations in thededicated relay method, and is unnecessary to repeat here.

[0061] The dual channel relay method may have several differentcombinations of channels for acknowledgements and data transmission. Thecentral coordinator may send data over a dedicated channel, apoint-to-point traffic channel, or a point-to-multipoint trafficchannel. Similarly, the destinations may acknowledge reception of thedata on a point-to-point traffic channel, or the point-to-pointdedicated channel between each device and the central coordinator. Thecentral coordinator may then communicate with the source device on apoint-to-point traffic channel or the point-to-point dedicated channel.

[0062] Having discussed four different methods to transmit data, a meansto evaluates them for broadcast/multicast and determine an “optimal”method becomes useful. Optimality is defined here will be from both anetwork perspective and from an application perspective. From thenetwork perspective, a broadcast/multicast transmission is said to beoptimal if the bandwidth required, as defined as number of symbols, forsuccessfully transmission is minimum. From the application perspective,a broadcast/multicast transmission is said to be optimal if the delay isminimized.

[0063] As discussed above, time is assumed to be slotted and slots areorganized into a Frame. Bandwidth allocations to a channel include theslots within a frame and the frequencies/tones the channel can use. Theallocation also includes the duration for which the channel might usethese slots/tones. Several parameters will be used in analyzing thediscussed methods, as defined below.

[0064] L_(pkt): The length of the packet or burst that needs to betransmitted.

[0065] L_(req): The length of the request and response messages used toestablish a Traffic Channel.

[0066] P_(win): Probability of successful transmission using theContention Channel.

[0067] EI: Equipment Identifier is a globally unique identifier for alldevices in the network.

[0068] TI: Tone Identifier is a globally unique identifier for each toneor carrier in a multi-tone system such as OFDM.

[0069] T_(frame): Duration of a Time Frame.

[0070] T_(symbol): Duration of a single symbol.

[0071] M: Multicast set defined as the set of EIs of the destinationdevices that are the intended recipients of the multicast messagetransmission.

[0072] Br: Broadcast set defined as the set of EIs of the all activedevices in the network, except the source of transmission.

[0073] i: Variable used to denote source EIs. The variable i can takethe EI value of any active device in the network.

[0074] j: Variable used to denote destination EIs. The variable i cantake the EI value of any active device in the network. The variable jcan also take on the value M to denote a multicast set of recipients orBr to denote a broadcast set of destination EIs. The variable j takesthe value central coordinator when the destination is the CC.

[0075] k: Variable used to denote a Tone Identifier (TI).

[0076] l: Variable used to denote the type of channel. The variable lcan take the values D-CH, T-CH, C-CH to denote Dedicated Channel,Traffic Channel and Contention Channel respectively.

[0077] B_(min): Minimum modulation density defined as bits/symbolsupported by the physical transmission system.

[0078] B_(max): Maximum modulation density defined as bits/symbolsupported by the physical transmission system.

[0079] A^(i,j,l): Allocation or bandwidth assignment to a particularchannel. A^(i,j,l) is a data structure with the following informationelements: ${A^{i,j,l} = \begin{Bmatrix}{i,{{EI}\quad {of}\quad {the}\quad {Source}\quad {Device}}} \\{j,{{EIs}\quad {of}\quad {the}\quad {Destination}\quad {Devices}}} \\\begin{matrix}\quad & \quad & {{j = {M\quad {for}\quad {multicast}\quad {set}}},{j = {{Br}\quad {for}\quad {Broadcast}\quad {set}}},} \\\quad & \quad & {j = {{CC}\quad {for}\quad {Central}\quad {Coordinator}}}\end{matrix} \\{l,{{Type}\quad {of}\quad {channel}}} \\\begin{matrix}\quad & \quad & {l = {D\quad C\quad {for}\quad {Dedicated}\quad {Channel}}} \\\quad & \quad & {l = {{TC}\quad {for}\quad {Traffic}\quad {Channel}}} \\\quad & \quad & {l = {{CCh}\quad {for}\quad {Contention}\quad {Channel}}}\end{matrix} \\{T_{alloc},{{Duration}\quad {of}\quad {the}\quad {allocation}\quad {within}\quad a\quad {Frame}}} \\\begin{matrix}{\Phi^{i,j,l},{{Set}\quad {of}\quad {TIs}\quad {identifying}\quad {tones}\quad {used}\quad {by}}} \\{{{channel}\quad l\quad {in}\quad {allocation}\quad A^{i,j,l}}}\end{matrix} \\{N_{tones}^{i,j,l},{{Number}\quad {of}\quad {tones}\quad {on}\quad {the}\quad {set}\quad \Phi^{i,j,l}}} \\{B_{k}^{i,j,l},{{Modulation}\quad {density}\quad {on}\quad {tone}\quad k\quad {in}\quad {allocation}\quad A^{i,j,l}}} \\\begin{matrix}{{B^{i,j,l} = \left\{ B_{k}^{i,j,l} \right\}},{{Set}\quad {of}\quad {modulation}\quad {density}\quad B_{k}^{i,j,l}\quad {on}}} \\{{{all}\quad {Tones}\text{/}{Carriers}\quad {to}\quad {be}\quad {used}\quad {by}}} \\{{{source}\quad {device}\quad i\quad {in}\quad {allocation}\quad A^{i,j,l}}}\end{matrix}\end{Bmatrix}},$

[0080] The optimality of each method will be determined by at least oneparameter of merit. A first parameter of merit is the delay. This isdefined as the delay (D_(method)) incurred in successful transmission ofa packet of length L_(pkt) from a source device to all intendeddestination devices, which might be a single device identified by EIj,or a set of EIs of devices defined by the Multicast set M or the set ofEIs of all devices in the network defined by the Broadcast set Br.

[0081] A second parameter of merit is the Bandwidth (BW_(method)) insymbols required for successful transmission of a packet of lengthL_(pkt) from a source device to all intended destination devices, whichmight be a single device identified by EIj, or a set of EIs of devicesdefined by the Multicast set M or the set of EIs of all devices in thenetwork defined by the Broadcast set Br. Each method has a differentcomputation to determine the bandwidth. The determination of the meritparameters may differ for each of the four transmission methodsdiscussed above: the contention access method; the direct broadcastmethod; the direct relay method and the dual channel relay methods. Inaddition the determination of the merit parameter may differ within eachmethod based upon the nature of the transmission, either broadcast ormulticast transmission. For contention access broadcast transmissions,the bandwidth required and delay incurred are given by: $\begin{matrix}{{BW}_{1}^{Br} = {\frac{1}{P_{win}} \times \left\lceil \frac{L_{pkt}}{N_{tones} \times B_{\min}} \right\rceil \times N_{tones}}} \\{D_{1}^{Br} = {\frac{1}{P_{win}} \times \left( {T_{frame} + {T_{symbol} \times \left\lceil \frac{L_{pkt}}{N_{tones} \times B_{\min}} \right\rceil}} \right)}}\end{matrix}$

[0082] For multicast transmissions, the bandwidth required in symbolsand delay incurred are given by: $\begin{matrix}{{BW}_{1}^{M} = {BW}_{1}^{Br}} \\{D_{1}^{M} = D_{1}^{Br}}\end{matrix}$

[0083] For the direct broadcast method, broadcast transmissions thebandwidth required and delay incurred are given by: $\begin{matrix}{{BW}_{2}^{Br} = {{\left\lceil \frac{L_{pkt}}{\sum\limits_{k \in \Phi^{i,{Br},{TC}}}B_{k}^{i,{Br},{TC}}} \right\rceil \times N_{tones}^{i,{Br},{TC}}} + {2*\left\lceil \frac{L_{req}}{\sum\limits_{k \in \Phi^{i,{CC},{D\quad C}}}B_{k}^{i,{CC},{D\quad C}}} \right\rceil*N_{tones}^{i,{CC},{D\quad C}}}}} \\{D_{2}^{Br} = {\frac{T_{frame}}{2} + {T_{symbol} \times \left\lceil \frac{L_{pkt}}{\sum\limits_{k \in \Phi^{i,{Br},{TC}}}B_{k}^{i,{Br},{TC}}} \right\rceil}}}\end{matrix}$

[0084] For multicast transmissions, the bandwidth required in symbolsand delay incurred are given by: $\begin{matrix}{{BW}_{2}^{M} = {{\left\lceil \frac{L_{pkt}}{\sum\limits_{k \in \Phi^{i,N,{TC}}}B_{k}^{i,M,{TC}}} \right\rceil \times N_{tones}^{i,M,{TC}}} + {2*\left\lceil \frac{L_{req}}{\sum\limits_{k \in \Phi^{i,{CC},{D\quad C}}}B_{k}^{i,{CC},{D\quad C}}} \right\rceil*N_{tones}^{i,{CC},{D\quad C}}}}} \\{D_{2}^{M} = {T_{frame} + {T_{symbol} \times \left\lceil \frac{L_{pkt}}{\sum\limits_{k \in \Phi^{i,M,{TC}}}B_{k}^{i,M,{TC}}} \right\rceil}}}\end{matrix}$

[0085] For dedicated relay broadcast transmissions, the bandwidthrequired and delay incurred are given by:${BW}_{3}^{Br} = {{\left\lceil \frac{L_{pkt}}{\sum\limits_{k \in \Phi^{i,{CC},{D\quad C}}}B_{k}^{i,{CC},{D\quad C}}} \right\rceil \times N_{tones}^{\quad {i,{CC},{D\quad C}}}} + {\sum\limits_{j \in {Br}}\left( {\left\lceil \frac{L_{pkt}}{\sum\limits_{k \in \Phi^{{CC},j,{D\quad C}}}B_{k}^{{CC},j,{D\quad C}}} \right\rceil \times N_{tones}^{{CC},j,{D\quad C}}} \right)}}$

[0086] The dedicated channel for each device might consist of a singletone operational throughout the time frame, or it might be multipletones operating only in the time period T_(alloc) within the frame.Therefore the delay is then given by:${{{{{If}\quad T_{alloc}} \geq {T_{symbol} \times \left\lceil \frac{L_{pkt}}{\sum\limits_{k \in \Phi^{i,{CC},{D\quad C}}}B_{k}^{i,{CC},{D\quad C}}} \right\rceil \quad {then}}},{D_{3}^{Br} = {{T_{symbol} \times \left\lceil \frac{L_{pkt}}{\sum\limits_{k \in \Phi^{i,{CC},{D\quad C}}}B_{k}^{i,{CC},{D\quad C}}} \right\rceil} + {\underset{j \in {Br}}{Max}\left( {\left\lceil \frac{L_{pkt}}{\sum\limits_{k \in \Phi^{{CC},j,{D\quad C}}}B_{k}^{{CC},j,{D\quad C}}} \right\rceil \times T_{symbol}} \right)}}}}{Else}},\quad {D_{3}^{Br} = \quad {\frac{T_{frame}}{2} + {\left\lceil \frac{T_{symbol} \times \left\lceil \frac{L_{pkt}}{\sum\limits_{k \in \Phi^{i,{CC},{D\quad C}}}B_{k}^{i,{CC},{D\quad C}}} \right\rceil}{T_{alloc}} \right\rceil \times T_{frame}} + {T_{symbol} \times \left\lceil \frac{L_{pkt}}{\sum\limits_{k \in \Phi^{i,{CC},{D\quad C}}}B_{k}^{i,{CC},{D\quad C}}} \right\rceil} + {\underset{j \in {Br}}{Max}\left( {\left\lceil \frac{T_{symbol} \times \left\lceil \frac{L_{pkt}}{\sum\limits_{k \in \Phi^{{CC},j,{D\quad C}}}B_{k}^{{CC},j,{D\quad C}}} \right\rceil}{T_{alloc}} \right\rceil \times T_{frame}} \right)} + {\underset{j \in {Br}}{Max}\left( {\left\lceil \frac{L_{pkt}}{\sum\limits_{k \in \Phi^{{CC},j,{D\quad C}}}B_{k}^{{CC},j,{D\quad C}}} \right\rceil \times T_{symbol}} \right)}}}$

[0087] For multicast transmissions, the bandwidth required in symbolsand delay incurred are given by:${BW}_{3}^{M} = {{\left\lceil \frac{L_{pkt}}{\sum\limits_{k \in \Phi^{i,{CC},{D\quad C}}}B_{k}^{i,{CC},{D\quad C}}} \right\rceil \times N_{tones}^{\quad {i,{CC},{D\quad C}}}} + {\sum\limits_{j \in M}\left( {\left\lceil \frac{L_{pkt}}{\sum\limits_{k \in \Phi^{{CC},j,{D\quad C}}}B_{k}^{{CC},j,{D\quad C}}} \right\rceil \times N_{tones}^{{CC},j,{D\quad C}}} \right)}}$

[0088] The Dedicated Channel for each device might consist of a singletone operational throughout the time frame, or it might be multipletones operating only in the time period T_(alloc) within the frame.Therefore the delay is then given by:${{{If}\quad T_{alloc}} \geq {T_{symbol} \times \left\lceil \frac{L_{pkt}}{\sum\limits_{k \in \Phi^{i,{CC},{D\quad C}}}B_{k}^{i,{CC},{D\quad C}}} \right\rceil \quad {then}}},\text{}{D_{3}^{Br} = {\frac{T_{frame}}{2} + {T_{symbol} \times \left\lceil \frac{L_{pkt}}{\sum\limits_{k \in \Phi^{i,{CC},{D\quad C}}}B_{k}^{i,{CC},{D\quad C}}} \right\rceil} + {\underset{j \in M}{Max}\left( {\left\lceil \frac{L_{pkt}}{\sum\limits_{k \in \Phi^{{CC},j,{D\quad C}}}B_{k}^{{CC},j,{D\quad C}}} \right\rceil \times T_{symbol}} \right)}}}$${Else},\quad {D_{3}^{Br} = {\frac{T_{frame}}{2} + {\left\lceil \frac{T_{symbol} \times \left\lceil \frac{L_{pkt}}{\sum\limits_{k \in \Phi^{i,{CC},{D\quad C}}}B_{k}^{i,{CC},{D\quad C}}} \right\rceil}{T_{alloc}} \right\rceil \times T_{frame}} + {T_{symbol} \times \left\lceil \frac{L_{pkt}}{\sum\limits_{k \in \Phi^{i,{CC},{D\quad C}}}B_{k}^{i,{CC},{D\quad C}}} \right\rceil} + {\underset{j \in {Br}}{Max}\left( {\left\lceil \frac{T_{symbol} \times \left\lceil \frac{L_{pkt}}{\sum\limits_{k \in \Phi^{{CC},j,{D\quad C}}}B_{k}^{{CC},j,{D\quad C}}} \right\rceil}{T_{alloc}} \right\rceil \times T_{frame}} \right)} + {\underset{j \in M}{Max}\left( {\left\lceil \frac{L_{pkt}}{\sum\limits_{k \in \Phi^{{CC},j,{D\quad C}}}B_{k}^{{CC},j,{D\quad C}}} \right\rceil \times T_{symbol}} \right)}}}$

[0089] For dual channel relay broadcast transmissions, the bandwidthrequired and delay incurred are given by:${BW}_{4}^{Br} = {{\left\lceil \frac{L_{pkt}}{\sum\limits_{k \in \Phi^{i,{CC},{D\quad C}}}B_{k}^{i,{CC},{D\quad C}}} \right\rceil \times N_{tones}^{i,{CC},{D\quad C}}} + {\left\lceil \frac{L_{pkt}}{\sum\limits_{k \in \Phi^{{CC},{BR},{D\quad C}}}B_{k}^{{CC},{Br},{TC}}} \right\rceil \times N_{tones}^{{CC},{Br},{TC}}}}$

[0090] The Dedicated Channel for each device might consist of a singletone operational throughout the time frame, or it might be multipletones operating only in the time period T_(alloc) within the frame.Therefore the delay is then given by:${{{If}\quad T_{alloc}} \geq {T_{symbol} \times \left\lceil \frac{L_{pkt}}{\sum\limits_{k \in \Phi^{i,{CC},{D\quad C}}}\quad B_{k}^{i,{CC},{D\quad C}}} \right\rceil \quad {then}}},{D_{4}^{Br} = {{T_{symbol} \times \left\lceil \frac{L_{pkt}}{\sum\limits_{k \in \Phi^{i,{CC},{D\quad C}}}\quad B_{k}^{i,{CC},{D\quad C}}} \right\rceil} + {T_{symbol} \times \left\lceil \frac{L_{pkt}}{\sum\limits_{k \in \Phi^{{CC},{BR},{D\quad C}}}\quad B_{k}^{{CC},{Br},{T\quad C}}} \right\rceil} + \frac{T_{frame}}{2}}}$${Else},{D_{4}^{Br} = {\frac{T_{frame}}{2} + {\left\lceil \frac{T_{symbol} \times \left\lceil \frac{L_{pkt}}{\sum\limits_{k \in \Phi^{i,{CC},{D\quad C}}}\quad B_{k}^{i,{CC},{D\quad C}}} \right\rceil}{T_{alloc}} \right\rceil \times T_{frame}} + {T_{symbol} \times \left\lceil \frac{L_{pkt}}{\sum\limits_{k \in \Phi^{i,{CC},{D\quad C}}}\quad B_{k}^{i,{CC},{D\quad C}}} \right\rceil} + {T_{symbol} \times \left\lceil \frac{L_{pkt}}{\sum\limits_{k \in \Phi^{{CC},{BR},{D\quad C}}}\quad B_{k}^{{CC},{Br},{T\quad C}}} \right\rceil}}}$

[0091] For multicast transmissions, the bandwidth required in symbols isgiven by:${BW}_{4}^{M} = {{\left\lceil \frac{L_{pkt}}{\sum\limits_{k \in \Phi^{i,{CC},{D\quad C}}}\quad B_{k}^{i,{CC},{D\quad C}}} \right\rceil \times N_{tones}^{i,{CC},{D\quad C}}} + {\left\lceil \frac{L_{pkt}}{\sum\limits_{k \in \Phi^{{CC},M,{D\quad C}}}\quad B_{k}^{{CC},M,{T\quad C}}} \right\rceil \times N_{tones}^{{CC},M,{TC}}}}$

[0092] For multicast transmissions, the delay symbols incurred incompleting the multicast is given by:${{{If}\quad T_{alloc}} \geq {T_{symbol} \times \left\lceil \frac{L_{pkt}}{\sum\limits_{k \in \Phi^{i,{CC},{D\quad C}}}\quad B_{k}^{i,{CC},{D\quad C}}} \right\rceil \quad {then}}},{D_{4}^{M} = {{T_{symbol} \times \left\lceil \frac{L_{pkt}}{\sum\limits_{k \in \Phi^{i,{CC},{D\quad C}}}\quad B_{k}^{i,{CC},{D\quad C}}} \right\rceil} + {T_{symbol} \times \left\lceil \frac{L_{pkt}}{\sum\limits_{k \in \Phi^{{CC},M,{D\quad C}}}\quad B_{k}^{{CC},M,{T\quad C}}} \right\rceil} + \frac{T_{frame}}{2}}}$${Else},{D_{4}^{M} = {\frac{T_{frame}}{2} + {\left\lceil \frac{T_{symbol} \times \left\lceil \frac{L_{pkt}}{\sum\limits_{k \in \Phi^{i,{CC},{D\quad C}}}\quad B_{k}^{i,{CC},{D\quad C}}} \right\rceil}{T_{alloc}} \right\rceil \times T_{frame}} + {T_{symbol} \times \left\lceil \frac{L_{pkt}}{\sum\limits_{k \in \Phi^{i,{CC},}}\quad B_{k}^{i,{CC},{D\quad C}}} \right\rceil} + {T_{symbol} \times \left\lceil \frac{L_{pkt}}{\sum\limits_{k \in \Phi^{{CC},M,{D\quad C}}}\quad B_{k}^{{CC},M,{T\quad C}}} \right\rceil}}}$

[0093] With these parameters of merit defined, it is now possible todetermine an optimal method of transmission. Governing this decision aresome simple rules. If a device needs to transmit a single burst orpacket, then the device uses the following algorithm to choose andcompute the optimality metric. If a device wishes to do continuousbroadcasts or multicasts, the same algorithm is used to determine theoptimal method and this method is used for all the broadcast/multicasttransmission, until key parameters such as the allocations to the T-CHor D-CH or the P_(win) parameter of the C-CH change. In the event ofsuch a change, the device or the central coordinator may re-compute themetrics and change the method if the current method proves sub-optimal.

[0094] The decision on the optimal method may be made either by thecentral coordinator or by the source device. In either case, both thedevice and central coordinator must have all the information required tomake the decision. A flowchart of one embodiment of making such adecision is shown in FIG. 7. The first stage in the process is theexchange of relevant information between the central coordinator and thesource device, which includes all parameters defined above that are usedin the computation of the parameters of merit. These defined parameterswill be referred to as the system parameters.

[0095] As can be seen in FIG. 7, the source device desiring transmissionwill inform the central coordinator of the intended devices for thetransmission, such as all devices for broadcast, or a defined group fora multicast at 140. At 142 the source device obtains the systemparameters, such as P_(win), the allocations A^(i,CC,D-CH) for DedicatedChannels from all devices (i) in the system, and a specific TrafficChannel allocation A^(i,Br/M,T-CH). The source device uses a localparameter at 144 such as the length of the packet, the length of aburst, if a burst is used instead of a packet, and the length of therequest and response messages used to establish the Traffic Channel. Thesource device also uses the information obtained in from the centralcoordinator to compute the delay and bandwidth figures of merit asdefined above. If the central coordinator is making the decision, thisinformation is communicated to the CC.

[0096] The relevant merit parameter is then determined at 144. Whichparameter is used is determined by the transmission requirements at 150.For example, if the source device needs to successfully transmit thepacket to all destination devices in minimum time then the devicechooses the method that has the lowest Delay parameter of merit. Thedelay parameter is computed for all four methods at 152and the one withthe lowest delay is selected at 146.

[0097] If the network chooses to minimize the amount of bandwidth insymbols that would be required for the packet transmission, thebandwidth figure of merit is computed for all methods by the device orthe central coordinator at 154 and the method with the least bandwidthvalue is chosen for the broadcast/multicast transmission at 146. Unlessthe allocations defined above change, the device will continue to usethe same method for broadcast/multicast as determined in by theappropriate parameter of merit.

[0098] These methods and processes may be implemented in software orhardware in the respective devices. A block diagram of an embodiment ofa device is shown in FIG. 8. As mentioned above, any device may act asthe central coordinator that has the processing capability. The device160 has a processor 164. In an alternative embodiment the device has twosuch processors. Within the processor 160 is the protocol stack 164,which includes software instructions to implement the transport, MAC andphysical layers, as well as the connection manager (CM). When the deviceis acting as the central coordinator, the software that implements theCBWM is active. When the device is not the central coordinator, the CBWMis inactive.

[0099] In addition to the processor or processors, other hardwarecomponents are made available for the MAC, transport and physical layerfunctions. This hardware may include memory registers, fieldprogrammable gate arrays (FPGAs), application specific integratedcircuits (ASICs), etc. The specific architecture and implementation ofthese components and their functions are left to the system designer.The device communicates through an array of ports, from port 1 172 toport N 174, where the exact number of ports is also left to the systemdesigner.

[0100] In the instance where the methods of the invention areimplemented in software in a pre-existing device with the necessaryhardware capabilities and capacity, the software may take the form of anarticle of machine-readable media. Software code resides on the media,and when executed by the processor, the software causes the machine ordevice to perform the processes and methods of the invention.

[0101] Thus, although there has been described to this point aparticular embodiment for a method and apparatus for transmitting datain a centralized network, it is not intended that such specificreferences be considered as limitations upon the scope of this inventionexcept in-so-far as set forth in the following claims.

What is claimed is:
 1. A method of determining a transmission process,the method comprising: notifying a central coordinator of a group toreceive a transmission; obtaining system parameters from the centralcoordinator; determining at least one merit parameter using at least oneof the system parameters and at least one local parameter; and selectinga method of transmission based upon the merit parameter.
 2. The methodof claim 1 wherein obtaining system parameters further comprisesobtaining at least one of the group comprised of: a probability oftransmission of a packet using a contention channel, allocation for anydedicated channels, and a traffic channel allocation.
 3. The method ofclaim 1, wherein determining at least one merit parameter furthercomprising using a length of a packet.
 4. The method of claim 1,wherein: determining at least one merit parameter further comprisesdetermining a delay merit parameter for at least two different methodsof transmission; and selecting a method of transmission furthercomprises selecting the method of transmission with a lowest delay meritparameter.
 5. The method of claim 1, wherein: determining at least onemerit parameter further comprises determining a bandwidth meritparameter for at least two different methods of transmission; andselecting a method of transmission further comprises selecting themethod of transmission with a lowest bandwidth merit parameter.
 6. Themethod of claim 1, the method further comprising setting the method oftransmission selected as a default method of transmission.
 7. The methodof claim 1, selecting a method of transmission further comprisingselecting one from the group comprising: a contention access method, adirect broadcast/multicast method, a dedicated relay method, and a dualchannel relay method.
 8. A device, comprising: a port to allow thedevice to communicate with a centralized network; a processor to: notifya central coordinator of a group to receive a transmission; obtain atleast one system parameter from the central coordinator; determine atleast one merit parameter using at least one system parameter and atleast one local parameter; and select a method of transmission basedupon the merit parameter.
 9. The device of claim 8, the centralizednetwork further comprising a power line communication system.
 10. Thedevice of claim 8, at least one system parameter further comprising oneselected from the group comprised of: a probability of transmission of apacket using a contention channel, allocation for any dedicatedchannels, and a traffic channel allocation.
 11. The device of claim 8,at least one local parameter further comprising one selected from thegroup comprised of: a length of a packet, a length of a burst, and alength of request and response messages used to establish a trafficchannel.
 12. The device of claim 8, the device further comprising aconnection manager.
 13. An article of machine-readable media containingcode that, when executed, causes the machine to: notify a centralcoordinator of a group to receive a transmission; obtain systemparameters from the central coordinator; determine at least one meritparameter using at least one of the system parameters and at least onelocal parameter; and select a method of transmission based upon themerit parameter.
 14. The article of claim 13, the code, when executed,further causing the machine to set the method of transmission selectedas a default method of transmission.